The high school I attended required two years of Latin and Philosophy in order to graduate, (and it was a public school!) At every assembly, we recited the Pledge in Latin (I've highlighted what you're looking for):

Yup, to translate it properly, you just need to consider what case it is. "To one nation", "in one nation", "with one nation", is just the matter of a preposition in English, but in Latin, the endings will be different. So... Will need the whole sentence to tell you what form to use.

Chevalier - This is from the Pledge of Allegiance...but it is part of a tattoo I'm having designed that brings together my love for both my country and God.

If you mean for it to be a quote from the Pledge of Allegiance, then Uni nationi Deo ducente is fine.

But know that, since Latin an inflected language, that phrase bears grammatical meaning as well as lexical meaning, viz (with the [bracketed part] being left out of the Latin): [I pledge allegiance] to one nation under GodThat is: It is the indirect object of the verb "pledge". Outside context it could also be translated as "for one nation..." or a handful of other grammatical structures.

If you don't want that grammatically-included "to" in there, use the nominative form: Una natio Deo ducente

You do want the inflected Deo ducente, since as both I and Roselander have pointed out, it really means "with God leading" ie "under God['s providence/authority/direction]"